JP5272548B2 - Manufacturing method of high strength cold-rolled steel sheet with low yield strength and small material fluctuation - Google Patents

Description

  The present invention relates to a method for producing a high-strength cold-rolled steel sheet for press forming that is used through a press forming process in automobiles, home appliances, and the like.

  Conventionally, automotive exterior panels that require dent resistance, such as hoods, doors, trunk lids, back doors, and fenders, are solidified by adding carbonitride-forming elements such as Nb and Ti based on ultra-low carbon steel. Tensile strength with controlled C content TS: 340MPa class BH steel plate (bake hardening type steel plate, hereinafter simply referred to as 340BH) and TS: 270MPa class IF steel plate (Interstitial Free steel plate, hereinafter simply referred to as 270IF) It has been. In recent years, due to further increasing needs for weight reduction of vehicle bodies, studies are being made to increase the strength of these outer panel panels to improve dent resistance and to reduce the thickness of steel sheets. In addition, studies are underway to improve the dent resistance by increasing the strength with the same thickness as the current state, and to reduce the temperature and time of the baking coating process to which BH is applied.

  However, if a steel plate is made thinner by adding solid solution strengthening elements such as Mn and P based on 340BH with a yield strength of YP: 230MPa and 270IF of YP: 180MPa, the problem of surface distortion occurs. . Here, the surface distortion is a fine wrinkle or a wavy pattern on the press-molded surface caused by an increase in YP. When this surface distortion occurs, the design and design of doors and trunk lids are significantly impaired. For this reason, in such applications, it is desired that the yield stress YP after press molding and baking coating is increased more than before, but has a YP as low as possible before press molding.

From such a background, for example, in Patent Document 1, the cooling rate after annealing of steel containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.023 to 0.8% is optimized, mainly ferrite. A method of obtaining a steel sheet having a low YP, a high work-hardening WH, and a high BH by forming a composite structure consisting of martensite is disclosed. Patent Document 2 contains C: 0.01 to 0.04%, Mn: 0.3 to 1.6%, Cr: 0.5% or less, Mo: 0.5% or less, and satisfies 1.3 ≦ Mn + 1.29Cr + 3.29Mo ≦ 2.1% Disclosed is a method for manufacturing a high-strength cold-rolled steel sheet having a high BH by annealing a steel, cooling at a temperature range of at least 550 ° C. at a cooling rate of 100 ° C./sec or more, increasing solid solution C in the steel. Has been. In Patent Document 3, after annealing steel containing C: 0.0025% or more and less than 0.04%, Mn: 0.5-2.5%, Cr: 0.05-2.0%, the temperature range of 650-450 ° C is 15-200 ° C / sec. Cooling at a cooling rate of 200 ° C, cooling at a temperature range near 200-300 ° C at a cooling rate of less than 10 ° C / sec, high BH consisting of ferrite and a low-temperature transformation phase and high surface quality after press molding A method for producing a strength cold-rolled steel sheet is disclosed.
Japanese Patent Publication No.62-40405 JP 2006-233294 A JP 2006-52465 A

However, the high-strength cold-rolled steel sheets manufactured by the manufacturing methods described in Patent Documents 1 to 3 have the following problems.
i) Low YP is not enough, and when it is press-molded on door panels, the amount of surface distortion is still large compared to 340BH.
ii) In such a high-strength cold-rolled steel sheet of the composite structure type, the second phase such as hard martensite is dispersed for strengthening, so that the material characteristics tend to fluctuate essentially. For example, the proportion of the second phase is significantly affected by the amount of C in the steel of several tens of ppm and the fluctuation of the annealing temperature of 20 to 50 ° C. Compared to 340BH and 270IF that are solid solution strengthened with conventional Mn and P Material fluctuation is large.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet having a sufficiently low YP and a small material fluctuation.

The present inventors have conducted intensive studies on a method for further reducing YP and simultaneously reducing material fluctuations while securing a high BH equivalent to or higher than that of conventional steel for composite-structured high-strength cold-rolled steel sheets. As a result, the following findings were found.
I) By appropriately controlling the composition range of Mn and Cr, and further gradually heating the predetermined temperature range during annealing, the second phase can be dispersed coarsely and uniformly, achieving a low YP, and against the annealing temperature. YP fluctuation can be kept small.
II) Further, by optimizing the composition range of Mn and Cr, an excessive decrease in the amount of dissolved C is suppressed, and high BH is obtained.

The present invention has been made based on the above knowledge, and as a component composition, by mass%, C: more than 0.01% and less than 0.08%, Si: 0.2% or less, Mn: 0.8% or more and less than 1.7%, P: 0.03% S: 0.02% or less, sol.Al: 0.3% or less, N: 0.01% or less, Cr: more than 0.4% and 2% or less, and further 1.9 <[Mneq] <3 and 0.34 ≦ [% Cr] / After satisfying [% Mn], the steel consisting of the remaining iron and inevitable impurities is hot-rolled and cold-rolled, and then heated at a temperature range of 680 to 740 ° C at an average heating rate of less than 3 ° C / sec. Tc given by the following formula (1) from 650 ° C by annealing at an annealing temperature of more than 820 ° C and less than 820 ° C, cooling the temperature range from the annealing temperature to 650 ° C at an average cooling rate of 2 to 30 ° C / sec. High strength, characterized in that the temperature range up to ℃ is cooled at an average cooling rate of 10 ℃ / sec or more, and the temperature range from Tc ℃ to 200 ℃ is cooled at an average cooling rate of 0.2 to 10 ℃ / sec A method for producing a cold-rolled steel sheet is provided.
Tc = 410-40 × [% Mn] -30 × [% Cr] ... (1)
Here, [Mneq] is Mn equivalent and represents [Mneq] = [% Mn] + 1.3 × [% Cr], and [% Mn] and [% Cr] represent the contents of Mn and Cr, respectively. .

  In the method for producing a high-strength cold-rolled steel sheet of the present invention, it is preferable to heat a temperature range of 680 to 740 ° C. at an average heating rate of less than 2 ° C./sec during annealing.

  Further, it is preferable to use steel satisfying 0.55 ≦ [% Cr] / [% Mn], or to further contain B: 0.005% or less by mass%. Further, it is preferable to contain at least one of Mo: 0.15% or less and V: 0.2% or less by mass%. Furthermore, it is preferable to contain at least one of Ti: less than 0.014%, Nb: less than 0.01%, Ni: 0.3% or less, and Cu: 0.3% or less in mass%.

  According to the present invention, a high-strength cold-rolled steel sheet having a low YP and a small material fluctuation can be produced. Moreover, since the high-strength cold-rolled steel sheet produced by the method of the present invention has excellent surface strain resistance and dent resistance, it is suitable for increasing the strength and thinning of automobile parts.

  Details of the present invention will be described below. Note that% representing the amount of a component means mass% unless otherwise specified.

1) Component composition
C: More than 0.01% and less than 0.08%
C is an element necessary for securing a predetermined amount of the second phase. If the amount of C added is too small, a sufficient amount of the second phase cannot be secured, and a low YP cannot be obtained. In addition, a sufficiently high BH cannot be secured, and at the same time, the aging resistance deteriorates. In order to secure a sufficient amount of the second phase, C needs to be added in excess of 0.01%. On the other hand, when the C content is 0.08% or more, the ratio of the second phase becomes too large and YP increases. Therefore, the upper limit of the C content is less than 0.08%. In order to obtain a lower YP, the C amount is preferably less than 0.06%, and in order to obtain a lower YP, the C amount is more preferably less than 0.04%.

Si: 0.2% or less
Addition of a small amount of Si has the effect of improving the surface quality by delaying the scale formation in hot rolling, the effect of making the microstructure of the steel sheet more uniform and coarse, and seizing on the mold during press forming (die Since there is an effect of improving galling, it can be added from such a viewpoint. However, since Si has a large solid solution strengthening ability and a large effect of increasing YP, the Si amount is set to 0.2% or less of the range where the influence of YP increase is small. Preferably it is 0.1% or less.

Mn: 0.8% or more and less than 1.7%
Mn enhances hardenability, and by optimizing the amount of Mn, the amount of solute C can be reduced to a predetermined range to reduce YP and increase BH. If the amount of Mn is less than 0.8%, the amount of dissolved C will increase too much during the cooling process during annealing, and a large amount of dissolved C will form around martensite when overaging is performed at a temperature range of less than 400 ° C. It becomes difficult to sufficiently reduce the YP by precipitating on the strain of the material. Moreover, when the amount of solute C increases too much, aging resistance will also deteriorate. On the other hand, when the amount of Mn is 1.7% or more, the amount of dissolved C becomes too small and BH decreases. Further, in addition to the increase in solid solution strengthening of Mn, the second phase becomes finer, leading to an increase in YP and fluctuation of YP with respect to the annealing temperature. Therefore, the Mn content is 0.8% or more and less than 1.7%.

P: 0.03% or less
P has a large solid solution strengthening amount, and it is better to reduce it as much as possible from the viewpoint of low YP. However, since it has the effect of coarsening the microstructure of the steel sheet and the effect of improving seizure to the mold during press forming, it can be added in a range of 0.03% or less, which has a small adverse effect on YP increase. .

S: 0.02% or less
S precipitates as MnS in the steel, but if the content is large, the ductility of the steel sheet is lowered and the press formability is lowered. Moreover, when hot-rolling a slab, hot ductility is reduced and surface defects are easily generated. For this reason, the amount of S is set to 0.02% or less, but the smaller the amount, the better.

sol.Al: 0.3% or less
Al is used as a deoxidizing element or as an element that improves the aging resistance by fixing N as AlN, but it forms fine AlN during coiling or annealing after hot rolling to increase ferrite grain growth. Suppress and inhibit low YP. From the viewpoint of reducing oxides in steel or improving aging resistance, Al should be added in an amount of 0.02% or more. On the other hand, from the viewpoint of improving the grain growth property, the grain growth property of ferrite is improved by setting the coiling temperature to 620 ° C. or higher. However, the smaller the fine AlN, the better. For this purpose, it is preferable that the amount of sol.Al is 0.15% or more and AlN is coarsely precipitated at the time of winding. However, if it exceeds 0.3%, the cost increases, so the amount of sol.Al is 0.3% or less. However, if sol.Al is added in excess of 0.1%, the castability is deteriorated and the surface quality is deteriorated. For outer panel applications that require strict control of the surface quality, sol.Al The amount is preferably 0.1 or less.

N: 0.01% or less
N precipitates at the time of winding or annealing after hot rolling to form fine AlN and inhibits grain growth. For this reason, the amount of N is set to 0.01% or less, but the smaller the amount, the better. Moreover, when N content increases, deterioration of aging resistance is caused. From the viewpoint of improving grain growth and aging resistance, the N content is preferably less than 0.008%, and more preferably less than 0.005%.

Cr: More than 0.4% and 2% or less
Cr is the most important element in the present invention, has a small amount of solid solution strengthening, has the effect of refining martensite, which is the second phase, and enhances hardenability, thus reducing YP and reducing material fluctuations. It is an effective element. In order to exert such effects, it is necessary to control the Mn equivalent and the composition ratio with Mn described below, but Cr needs to be added in excess of 0.4%. On the other hand, if the Cr content exceeds 2%, the cost increases and the surface quality of the plated steel sheet deteriorates, so the Cr content should be 2% or less.

1.9 <[Mneq] <3
The Mn equivalent defined in the present invention, that is, the above-mentioned [Mneq], by controlling the cooling rate during annealing and exceeding 1.9, the amount of solid solution C is reduced to an appropriate range, and pearlite and bainite are generated. Is suppressed and low YP and high BH are obtained. Furthermore, from the viewpoint of reducing YP, [Mneq] is desirably greater than 2.1, and more desirably greater than 2.2. On the other hand, if [Mneq] increases too much, BH decreases and costs increase, so [Mneq] is set to less than 3.

0.34 ≦ [% Cr] / [% Mn]
Even with the same [Mneq], the ratio of Cr amount to Mn amount, that is, [% Cr] / [% Mn] should be 0.34 or more to coarsen the second phase and reduce the solid solution strengthening of Mn, thus reducing YP. In addition, material fluctuation can be reduced. In order to further reduce YP and reduce material fluctuation, it is preferable to satisfy 0.55 ≦ [% Cr] / [% Mn].

  The balance is iron and inevitable impurities, but may further contain a predetermined amount of the following elements.

B: 0.005% or less
Similarly, B is an element that enhances hardenability, and also has an effect of improving grain growth by fixing N as BN. However, if B is added excessively, the second phase is reversely refined due to the effect of the remaining solid solution B, so the B content is preferably 0.005% or less. In the steel of the present invention, by adding more than 0.001% B, the effect of improving the grain growth property of ferrite is sufficiently exhibited, and an extremely low YP can be obtained. Therefore, it is desirable to contain B over 0.001%.

Mo: 0.1% or less
Mo, like Mn and Cr, is an element that enhances hardenability and can be added for the purpose of improving hardenability. However, if added excessively, the second phase is refined and hardened in the same manner as Mn to increase YP. Therefore, it is preferable to add Mo in a range of 0.1% or less that has a small effect on YP increase. From the viewpoint of further reducing YP and ΔYP, the Mo content is preferably less than 0.02% (no addition).

V: 0.2% or less
V is an element that similarly improves the hardenability, but if added over 0.2%, a significant cost increase is caused. Therefore, V is preferably added in a range of 0.2% or less.

Ti: Less than 0.014%
Ti has the effect of fixing N to improve aging resistance and castability. However, since fine precipitates such as TiN, TiC, and Ti (C, N) are formed in the steel to inhibit grain growth, the Ti content may be less than 0.014% from the viewpoint of low YP. preferable.

Nb: less than 0.01%
Nb has the effect of controlling the texture by delaying recrystallization in hot rolling and reducing YP in the rolling direction and the 45 degree direction. However, fine NbC and Nb (C, N) are formed in the steel to significantly deteriorate the grain growth and increase YP. Therefore, Nb should be contained within a range of less than 0.01%, which is less affected by YP increase. Is preferred.

Cu: 0.3% or less
Cu is an element mixed when scrap is actively used. By allowing Cu to be mixed, recycled materials can be used as raw materials, and manufacturing costs can be reduced. The influence of Cu on the material of the steel sheet is small, but if it is mixed excessively, it causes surface scratches, so the Cu content is preferably 0.3% or less.

Ni: 0.3% or less
Ni also has little influence on the material of the steel sheet, but Ni can be added from the viewpoint of reducing surface scratches when Cu is added. However, since Ni causes surface defects due to non-uniformity of scale when added in excess, the amount of Ni is preferably 0.3% or less.

2) Manufacturing conditions In the manufacturing method of the present invention, as described above, the steel slab having the above component composition is hot-rolled and cold-rolled, and then the temperature range of 680 to 740 ° C is less than 3 ° C / sec. Heat at an average heating rate, anneal at an annealing temperature of more than 740 ° C and less than 820 ° C, cool the temperature range from the annealing temperature to 650 ° C at an average cooling rate of 2 to 30 ° C / sec, from 650 ° C to the above Cool the temperature range up to Tc ° C given by equation (1) at an average cooling rate of 10 ° C / sec or more, and cool the temperature range from Tc ° C to 200 ° C at an average cooling rate of 0.2 to 10 ° C / sec. To do.

Hot rolling To hot-roll steel slabs, a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating, or rolling by subjecting the slab after continuous casting to a short heat treatment It can be done by the method to do. The hot rolling may be performed according to a conventional method, for example, the slab heating temperature is 1100 to 1300 ° C., the finish rolling temperature is not less than the Ar ₃ transformation point, the average cooling rate after finish rolling is 10 to 200 ° C./sec, The winding temperature may be 400 to 720 ° C. In order to obtain a beautiful plating surface quality for the outer panel, it is preferable that the slab heating temperature is 1200 ° C. or lower and the finish rolling temperature is 850 ° C. or lower. Further, it is preferable to sufficiently perform descaling in order to remove the primary and secondary scales generated on the steel plate surface. From the viewpoint of reducing YP, it is desirable that the coiling temperature be higher, and it is preferable to set the temperature to 640 ° C or higher. In particular, when the coiling temperature is 680 ° C. or higher, Mn and Cr can be sufficiently concentrated in the second phase in the hot-rolled sheet state, improving the stability of γ in the subsequent annealing process, and reducing the low YP Contributes to Also, in order to reduce the in-plane anisotropy of the r value of the steel sheet and to keep the YP in the rolling direction and 45 degrees low, the cooling rate after finish rolling can be increased to 40 ° C / sec or more. preferable.

Cold rolling In cold rolling, the rolling rate may be 50 to 85%.

Annealing Average heating rate during annealing: less than 3 ° C / sec In order to uniformly disperse the coarse second phase after annealing, to reduce YP and to reduce material fluctuations, in the temperature range of 680 ° C to 740 ° C It is effective to control the heating rate. This is because, in the component system with [Mneq] exceeding 1.9, the second phase after annealing tends to be refined, and since the Mn is high, the Ac ₁ transformation point becomes too low and recrystallization is not completed. This is because even if γ grains are formed at the crystal grain ferrite grain interface or recrystallization is completed, γ grains are formed in fine ferrite grains immediately after recrystallization, and the YP of the steel sheet is likely to increase.

  Steel containing C: 0.028%, Si: 0.01%, Mn: 1.6%, P: 0.01%, S: 0.01%, sol.Al: 0.04%, Cr: 0.8%, N: 0.003% was melted in the laboratory. To produce a 27 mm thick slab. This slab was heated to 1250 ° C., hot-rolled to 2.3 mm at a finish rolling temperature of 830 ° C., and subjected to a winding process at 620 ° C. for 1 hour. The obtained hot-rolled sheet was cold-rolled at a rolling rate of 67% to 0.75 mm. When annealing the obtained cold-rolled sheet, the average heating rate in the temperature range of 680-740 ° C is changed to 0.3-20 ° C / sec, soaking is performed at 780 ° C x 40 sec, from the annealing temperature to 650 ° C Is cooled at an average cooling rate of 7 ° C / sec, the temperature range from 650 ° C to 300 ° C is cooled at 25 ° C / sec, and then the temperature range from 300 ° C to 200 ° C is cooled at 0.5 ° C / sec. And air cooled to room temperature. A JIS No. 5 tensile specimen was collected from the obtained steel sheet, subjected to a tensile test (based on JIS Z2241, the tensile direction is perpendicular to the rolling direction), and the structure was observed by SEM.

  FIG. 1 shows the relationship between YP and the average heating rate in the temperature range of 680 to 740 ° C. during annealing. An YP of 200 MPa or less is obtained at an average heating rate of less than 3 ° C / sec, and an YP of 195 MPa or less is obtained at a heating rate of less than 2 ° C / sec. At this time, it was confirmed by SEM that the second phase was coarser and uniformly dispersed. Furthermore, the influence on the material fluctuation was investigated for the steel sheets annealed at various heating rates. That is, for each steel sheet, the annealing temperature was changed from 760 to 810 ° C., and the amount of YP variation ΔYP when the annealing temperature was changed by 50 ° C. was investigated. The heating rate at 680 to 740 ° C. during annealing was 20 ° C. / It was found that ΔYP was 20 MPa for the sec sample, whereas ΔYP was reduced to less than 15 MPa for the steel sheet with a heating rate of less than 3 ° C./sec. Thus, by controlling the heating rate within a predetermined temperature range, a steel sheet having a low YP and a small ΔYP with respect to the annealing temperature can be obtained.

Annealing temperature: More than 740 ° C and less than 820 ° C If the annealing temperature is 740 ° C or less, the solid solution of carbide becomes insufficient and the second phase cannot be secured stably. Above 820 ° C, the proportion of γ during annealing becomes too high, and the concentration of elements such as Mn and C into γ becomes insufficient, and a sufficiently low YP cannot be obtained. This is thought to be because element concentration to γ is insufficient, so that sufficient strain is not imparted around martensite and pearlite and bainite transformations are likely to occur in the cooling process after annealing. The soaking time may be 20 seconds or more in a temperature range exceeding 740 ° C., which is carried out by normal continuous annealing, and more preferably 40 seconds or more.

Average cooling rate in the temperature range from annealing temperature to 650 ° C (primary cooling rate): 2-30 ° C / sec
It is necessary to increase the average cooling rate in the temperature range from the annealing temperature to 650 ° C to 2-30 ° C / sec in order to condense Mn and C in the γ grains during cooling to improve hardenability and reduce YP. .

Average cooling rate (secondary cooling rate) in the temperature range from 650 ° C to Tc ° C given by equation (1) above: 10 ° C / sec or more Expressed from 650 ° C to Tc ° C, where pearlite and bainite are easily generated By cooling the temperature range near the Ms point at an average cooling rate of 10 ° C./sec or more, formation of pearlite and bainite is suppressed, and a sufficiently low YP can be obtained.

Average cooling rate in the temperature range from Tc ° C to 200 ° C (third cooling rate): 0.2 to 10 ° C / sec
By cooling the temperature range from Tc ° C to 200 ° C at an average cooling rate of 0.2 to 10 ° C / sec, excessive solute C remaining in the ferrite can be precipitated to reduce YP and increase ductility. it can.

  The high-strength cold-rolled steel sheet produced by the production method of the present invention has an YPEl of less than 0.5% and a sufficiently low YP in the annealed state, and can be used as it is as a press-forming steel sheet. However, normal skin pass rolling may be performed from the viewpoint of stabilizing press formability such as adjustment of surface roughness and flattening of the plate shape. In that case, the elongation is preferably 0.3 to 0.5% from the viewpoint of low YP, high El, and high WH.

  Steels Nos. A to BB shown in Table 1 were melted and continuously cast into 230 mm thick slabs. After heating this slab to 1180-1250 ° C, finish rolling temperature of 830 ° C (steel numbers A to D, I, R to V, X to BB), 880 ° C (steel numbers E to H, J to Q, W) And then hot rolled. Then, it cooled at the average cooling rate of 20 degreeC / sec, and wound up with the coiling temperature of 540-640 degreeC. The obtained hot-rolled sheet was cold-rolled at a rolling rate of 67 to 78% after pickling to obtain a cold-rolled sheet having a thickness of 0.75 mm. The obtained cold-rolled sheet has an average heating rate in the temperature range of 680 to 740 ° C. shown in Tables 2 and 3, an annealing temperature, a primary average cooling rate in the temperature range from the annealing temperature to 650 ° C., from 650 ° C. Annealing was performed at a secondary average cooling rate in the temperature range from Tc ° C to a tertiary average cooling rate in the temperature range from Tc ° C to 200 ° C. From the obtained annealed steel sheet, that is, not subjected to skin pass rolling, a JIS No. 5 test piece was taken from the direction perpendicular to the rolling direction and subjected to a tensile test (based on JIS Z2241) to evaluate YP and TS. Further, the difference between the maximum value and the minimum value of YP when the annealing temperature was changed in the range of 760 to 810 ° C. with respect to the steel sheets having the respective component compositions was determined as the YP fluctuation amount ΔYP. Further, 2% pre-strain was applied to the same test piece as described above, and BH, which was an increase in YP after heat treatment at 170 ° C. for 20 minutes, was obtained.

  The results are shown in Tables 2 and 3.

  The steel sheet of the example of the present invention has a low YP, that is, a low YR, as compared with a material having the same TS level. Moreover, ΔYP with respect to the annealing temperature is small, and the stability of YP is excellent. In particular, in steel sheets with [Mneq] exceeding 2.1, [% Cr] / [% Mn] optimized to 0.55 or more, and the heating rate during annealing controlled to less than 3 ° C / sec, Mn and solid Solid solution strengthening due to dissolved C is reduced, and the second phase is uniformly coarsened, so that YP is low and ΔYP is also small. For example, [Mneq] increases in steel numbers B, C, and D with respect to steel number A, but [% Cr] / [% Mn] is in the range of 0.34 to 0.41, so [Mneq] With the increase, the formation of pearlite and bainite is suppressed, and the solid solution C is reduced. The ΔYP with respect to the annealing temperature is in the range of 7 to 9 MPa. On the other hand, steel numbers A, B, C for steel numbers E, F, G, H, etc. with [Mneq] increased to over 2.1 and [% Cr] / [% Mn] adjusted to 0.55 or more Under the same production conditions as D, YP is 172 to 188 MPa, and ΔYP with respect to the annealing temperature is 4 to 6 MPa, which is very low. In addition, the increase in YP when C is increased is very small. Steel No. K with C increased to 0.058% has a very low YP of 208 MPa against TS of 490 MPa. Steel No. L with C increased to 0.072% has a very low YP of 230 MPa against TS of 541 MPa. That is, in this steel, even if C is changed, a steel plate having a small ΔYP and a low YR can be obtained stably. Furthermore, since the composition range of Mn and Cr is optimized, it has a high BH despite a low YP.

  In contrast, [Mneq], a steel whose heating rate and cooling rate during annealing are not optimized, has a higher YR than the steel of the same TS class. Even if [Mneq] is within the specified range, the steels with steel numbers S and V, where [% Cr] / [% Mn] is not optimized, are fine in the second phase and have a large Mn solid solution strengthening amount. Both ΔYP and YP are high. BH is also low. In steel No. T to which Mo is added, the second phase tends to become finer, YP is high, and ΔYP is large. As a result, low YR cannot be obtained with steel No. U in which the amount of C is not within the predetermined range and, as a result, the ratio of the second phase is not within the predetermined range. In steels Nos. X and Y with a large amount of P and Si added, the second phase is coarsened, but the amount of solid solution strengthening becomes too large, and a low YP cannot be obtained. As described above, the conventional steel cannot obtain a steel plate having all of low YP, small ΔYP, and high BH.

The figure which shows the relationship between the average heating rate at the time of annealing, and YP.

Claims (6)

As component composition, mass%, C: more than 0.01% and less than 0.08%, Si: 0.2% or less, Mn: 0.8% or more and less than 1.7%, P: 0.03% or less, S: 0.02% or less, sol.Al: 0.3% In the following, N: 0.01% or less, Cr: more than 0.4% and 2% or less, further satisfying 1.9 <[Mneq] <3 and 0.34 ≦ [% Cr] / [% Mn], from the remaining iron and inevitable impurities After the steel to be hot-rolled and cold-rolled, it is heated at an average heating rate of less than 3 ° C / sec in the temperature range of 680 to 740 ° C, and annealed at an annealing temperature of more than 740 ° C and less than 820 ° C. The temperature range from temperature to 650 ° C is cooled at an average cooling rate of 2 to 30 ° C / sec, and the temperature range from 650 ° C to Tc ° C given by the following formula (1) is average cooling of 10 ° C / sec or more. Cooling at a rate, and cooling the temperature range from Tc ° C. to 200 ° C. at an average cooling rate of 0.2 to 10 ° C./sec;
Tc = 410-40 × [% Mn] -30 × [% Cr] ... (1)
Here, [Mneq] is Mn equivalent and represents [Mneq] = [% Mn] + 1.3 × [% Cr], and [% Mn] and [% Cr] represent the contents of Mn and Cr, respectively. . 2. The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein a temperature range of 680 to 740 ° C. is heated at an average heating rate of less than 2 ° C./sec during annealing. 3. The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein a steel satisfying 0.55 ≦ [% Cr] / [% Mn] is used. 4. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein a steel containing B: 0.005% or less by mass% is used. The high-strength cold-rolled steel sheet according to any one of claims 1 to 4, wherein the steel contains at least one of Mo: 0.1% or less and V: 0.2% or less in mass%. Manufacturing method. Furthermore, using steel containing at least one of Ti: less than 0.014%, Nb: less than 0.01%, Ni: 0.3% or less, and Cu: 0.3% or less in mass%. 5. A method for producing a high-strength cold-rolled steel sheet according to any one of 5 above.

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